Fast fourier transform with down sampling based navigational satellite signal tracking

Abstract

A method and device to track navigational satellite signals, are claimed. In this invention, a combination of down-sampling and frequency domain transformation are used to track the navigational satellite signals under dynamic environment. A Fast Fourier Transform (FFT) with long coherent integration has been employed to determine the varying frequency components with high resolution. By representing a number of correlation values with their average value, it is possible to represent a long sequence of input values by a smaller number of values and a relatively short length FFT can reveal the low frequency components that are present in the signal during tracking operation. A large reduction in the computational load may be achieved using this down-sampling method without compromising on the frequency resolution.

The invention claimed is: 
     
       1. A method for tracking received signals in a global satellite navigation receiver, the method comprising:
 collecting a sequence of correlation values derived from the received signals correlated with PN codes: 
 partitioning the collected sequence into N groups of consecutive correlation values, where N is a positive integer; 
 calculating from each of the N groups, a corresponding set of N values forming a downsampled sequence; 
 converting the down-sampled sequence from time domain to frequency domain to produce a frequency domain sequence; 
 identifying a maximum power value in the frequency domain sequence; 
 calculating a frequency error between the received signals and a locally generated replica signal based on the identified maximum power value; and 
 adjusting a local replica carrier frequency by an amount equal to the frequency error. 
 
     
     
       2. The method of  claim 1 , wherein calculating the corresponding set of N values comprises averaging the consecutive correlation values in each of the N groups. 
     
     
       3. The method of  claim 1 , wherein calculating the corresponding set of N values comprises taking the root mean square of the consecutive correlation values in each of the N groups. 
     
     
       4. The method of  claim 1 , further comprising varying the value of N according to platform dynamics. 
     
     
       5. The method of  claim 1 , wherein the N values in the down-sampled sequence are assigned to temporal midpoints of the N groups. 
     
     
       6. The method of  claim 1 , wherein collecting the sequence of correlation values comprises selecting a total number of values to collect as a function of the receiver dynamics. 
     
     
       7. The method of  claim 1 , wherein adjusting the local replica carrier frequency comprises reducing a rate of adjustment to avoid spontaneous burst-like changes in the local replica carrier frequency due to signal noise. 
     
     
       8. The method of  claim 1 , further comprising fine-tuning the frequency error by interpolation. 
     
     
       9. The method of  claim 1 , further comprising employing a Frequency Lock Loop (FLL) to further improve the tracking performance. 
     
     
       10. The method of  claim 9 , wherein the frequency error obtained by FFT analysis is used as an input residual frequency to the Frequency Lock Loop (FLL). 
     
     
       11. The method of  claim 1 , wherein partitioning the collected sequence comprises aligning transitions between the groups with data bit edge transitions. 
     
     
       12. The method of  claim 1 , comprising selecting a number of frequency bins as a function of receiver dynamics. 
     
     
       13. The method of  claim 1 , wherein converting the down-sampled sequence from time domain to frequency domain comprises using a maximum likelihood criterion based method to remove data modulation. 
     
     
       14. A global satellite navigation system receiver device comprising an RF front end for receiving signals from navigational satellites, a baseband section receiving IF signals from the RF front end, and a processor receiving I, Q correlation values from the baseband section, wherein the processor:
 collects a sequence of the correlation values derived from the received signals correlated with PN codes; 
 partitions the collected sequence into N groups of consecutive correlation values, where N is a positive integer; 
 calculates from each of the N groups, a corresponding set of N values forming a downsampled sequence; 
 converts the down-sampled sequence from time domain to frequency domain to produce a frequency domain sequence; 
 identifies a maximum power value in the frequency domain sequence; 
 calculates a frequency error between the received signal and a locally generated signal based on the identified maximum power value; and 
 adjusts a local replica carrier frequency by an amount equal to the frequency error. 
 
     
     
       15. The device of  claim 14 , wherein calculating the corresponding set of N values comprises averaging the consecutive correlation values in each of the N groups. 
     
     
       16. The device of  claim 14 , wherein calculating the corresponding set of N values comprises taking the root mean square of the consecutive correlation values in each of the N groups. 
     
     
       17. The device of  claim 14 , further comprising varying the value of N according to platform dynamics. 
     
     
       18. The device of  claim 14 , wherein the N values in the down-sampled sequence are assigned to temporal midpoints of the N groups. 
     
     
       19. The device of  claim 14 , wherein collecting the sequence of correlation values comprises selecting a total number of values to collect as a function of the receiver dynamics. 
     
     
       20. The device of  claim 14 , wherein adjusting the local replica carrier frequency comprises reducing a rate of adjustment to avoid spontaneous burst-like changes in the local replica carrier frequency due to signal noise. 
     
     
       21. The device of  claim 14 , further comprising fine-tuning the frequency error by interpolation. 
     
     
       22. The device of  claim 14 , further comprising employing a Frequency Lock Loop (FLL) to further improve the tracking performance. 
     
     
       23. The device of  claim 22 , wherein the frequency error obtained by FFT analysis is used as an input residual frequency to the Frequency Lock Loop (FLL). 
     
     
       24. The device of  claim 14 , wherein partitioning the collected sequence comprises aligning transitions between the groups with data bit edge transitions. 
     
     
       25. The device of  claim 14 , comprising selecting a number of frequency bins as a function of receiver dynamics. 
     
     
       26. The device of  claim 14 , wherein converting the down-sampled sequence from time domain to frequency domain comprises using a maximum likelihood criterion based method to remove data modulation.